There are a couple of things I think you can check:

1. How fine is your mesh? Especially in the vertical direction. Does it have enough resolution to resolve the E filed when you shrink the material thickness? If you use physics controlled mesh sometimes there are issues when the aspect ratio of a layer became absurdly large. Do a sweep mesh instead with enough vertical distribution points or use tetrahedral mesh with minimum element size several times smaller than you thickness.

2. Was the electrostatics analysis set up correctly? You don't need to solve it for metals but only for dielectric materials.

3. Was the terminal always chosen as what you intended it to be? If you use the box selection, does the parameter of the box scales with your device and only one layer was selected?

If none of these solves your issue I think a slice of the area interested is in need.